class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode prehead = new ListNode(-1);

        ListNode prev = prehead;
        while(l1 != null && l2 != null) {
            if(l1.val > l2.val) {
                prev.next = l2;
                l2 = l2.next;
            }else {
                prev.next = l1;
                l1 = l1.next;
            }
            prev = prev.next;
        }
        prev.next = l1 == null ? l2:l1;
        return prehead.next;
    }
}